Imaging enhancements by polarized illumination: theory and experimental verification

The polarization properties of light become more and more important as numerical apertures of the projection lens increase. With unpolarized light the contrast of the image is degraded because of poor interference of the TM component of the light. By applying only TE linear polarized illumination light, the contrast loss can be minimized. The challenge will be to control the polarization variation throughout the imaged field. Besides contrast also the light incoupling in the resist depends on polarization. The different polarization directions (TE and TM) induce virtual dose differences. Immersion lithography reduces this effect due to reduced incident angles at a given lens NA. In the upcoming era beyond 0.9 NA, imaging enhancements by polarized illumination are needed. There are several components in a lithographic scanner which potentially influence polarization properties. Apart from illuminator and projection lens the reticle blank and the patterned mask absorber including 3D effects may impact the final intensity distribution in the resist. Last but not least the ability to measure the polarization state is a prerequisite to actively control polarization within the exposure system. The ability to assess the unpolarized and polarized projection lens performance with the on-scanner interferometer (ILIASTM) allows us to do this. In order to verify the benefits and challenges of polarized illumination systems, we built a prototype illuminator and tested it on both a 0.85 NA ArF system as well as on a 0.93 NA ArF system. Next to the successful qualification of illuminator and projection lens we were able to verify the expected gain in imaging performance with polarized light. In this paper we present results of the experimental work and compare the data with our simulations.